Federal Reserve officials are increasingly concerned about the potential risks of its asset purchases on financial markets, but look set to continue its open-ended stimulus program for now.

Minutes from the Fed's December policy meeting showed a growing reticence about further increases in the central bank's $2.9 trillion balance sheet, which it expanded sharply in response to the financial crisis and recession of 2007-2009.

"Several (officials) thought that it would probably be appropriate to slow or to stop purchases well before the end of 2013, citing concerns about financial stability or the size of the balance sheet," the minutes said.

Wall Street picked up on the report's hawkish tone, with stock prices drifting lower after the announcement, while the dollar extended gains against the euro.

Still, the Fed appeared likely to continue buying assets for the foreseeable future, having announced in December it was extending monthly purchases of $40 billion in mortgage securities and also buying $45 billion in Treasuries each month.

A few of the voting members on the central bank's policy-setting Federal Open Market Committee thought asset buying would be warranted until about the end of 2013. A few others highlighted the need for further large-scale stimulus but did not specify an amount or time frame.

Fed officials generally agreed that the labor market outlook was not likely to improve without further nudging from the monetary authorities.

The U.S. economy expanded a respectable 3.1 percent in the third quarter on an annualized basis, but growth is believed to have slowed sharply to barely above 1 percent in the last three months of the year.

Data on Thursday showed a solid gain of 215,000 new private sector jobs for December, while analysts polled by Reuters last week were looking for a rise of 150,000 new jobs in the Labor Department's official survey, due out on Friday.

In the December meeting, the Fed also launched a new framework of policy thresholds, numerical guideposts that are supposed to give markets and the public a clearer idea of how policymakers will react to incoming economic data.

Officials say they will keep interest rates near zero until the unemployment rate falls to 6.5 percent for as long as estimates of medium-run inflation do not exceed 2.5 percent.

The minutes suggested it took officials some time to build a consensus around the idea.

"A few participants expressed a preference for using a qualitative description of the economic indicators influencing the Committee's thinking," the minutes said.

U.S. unemployment has come down steadily after hitting a peak of 10 percent in late 2009, but remains elevated at 7.7 percent.